Current active fixation pacing and ICD leads typically employ a tissue fixation helical anchor. Such helical anchors are well known in the art, are the most utilized fixation configuration in cardiac leads, and have been utilized in implantable cardiac leads in cardiac rhythm disease management systems for over 30 years.
A helical anchor is typically located in the distal end of the lead and is mechanically coupled to a shaft within a distal portion of the lead. The shaft is mechanically coupled to the inner coil conductor, which extends through the lead to the lead connector end at the lead proximal end, the lead connector end being configured to mechanically couple to a pulse generator such as, for example, a pacemaker or implantable cardioverter defibrillator (“ICD”). A pin contact extending proximally from the lead connector end is mechanically coupled to the inner coil conductor. The helical anchor, shaft, inner conductor coil and pin contact are rotatable as a unit relative to the rest of the lead and lead connector end.
A helical anchor is typically deployed (i.e., extended from within the lead distal end) and fixated into the cardiac tissue by rotation of the pin contact relative to the rest of the lead connector end in a direction such as, for example, a clockwise direction. Thus, when it is desired to achieve fixation of the lead distal end to the implantation site, the physician rotates the connector pin clockwise relative to the rest of the lead connector end, thereby causing the helical anchor to extend out of the lead distal end and screw into the tissue.
An ongoing difficulty with active fixation leads employing helical anchors a physician has no reliable way to determine exactly when the helical anchor is fully embedded into the cardiac tissue at the desired implantation site. Since it is not possible to determine via fluoroscopy when the helical anchor is fully embedded into the cardiac tissue, it is common for a physician to fail to fully embed the helical anchor into the cardiac tissue. Indeed, only a portion of the helical anchor may actually be embedded, resulting in a high risk for dislodgement of the helical anchor from the tissue, which would then render the lead incapable of delivering the intended electrical therapy. Additionally, a helical anchor electrode that is not fully embedded into the cardiac tissue may also cause increased tissue damage to the local tissue adjacent to the helical anchor electrode, thereby resulting in deterioration of the electrical parameters (e.g., pacing capture thresholds, sensing of cardiac signals, etc.).
The opposite situation can also occur and cause therapy malfunction. That is, the physician, not knowing if the helical anchor is fully embedded into the desired cardiac tissue, may either over-rotate the connector pin numerous additional rotations (“over-torqueing the helical anchor”) or may rotate the whole lead body to try and “tighten-up” the helical anchor into the tissue. The result of such “over-torqueing” can be damage to the lead and, more specifically, the connections (e.g., welds, etc.) between the pin contact and inner conductor coil, the inner conductor coil and the shaft, or the shaft and the helical anchor. The result of such “over-torqueing” can also be compression of the tissue initially, subsequent coring of the cardiac tissue by the helical anchor, much like a wine bottle corkscrew can “chew-up” the cork when it is over rotated into the cork and the handle is bottomed-out against the bottle top. This compression and ultimately the “coring” of cardiac tissue at an implantation site with via a helical anchor of a lead is well described by T. Laske, et. al; in PACE, September, 2005, pg 887. Such coring can significantly increase the risk for damaged tissue at the helical anchor site, resulting in deterioration of the lead's electrical parameters and ultimately the lead's distal tip has a much higher risk of perforating the heart wall.
There is a need in the art for a lead configured to allow for determining whether or not the helical anchor is fully embedded in the cardiac tissue of the desired implantation site.